1. Field of the Invention
The present invention relates to a system and method of correcting the outputs of gyroscopes for force-dependent errors. More specifically, one embodiment of the invention pertains to a calibration method for a gyroscope that utilizes a force-effect model.
2. Description of Related Art
Aircraft inertial navigation relies upon the integration of data throughout a sequence that begins when the aircraft is prepared for takeoff and ends when the aircraft has landed and motion ceased. The inertial navigation system (“INS”) of an aircraft includes various components, including accelerometers and gyroscopes, that convert the effects of inertial forces into acceleration, velocity and position measurements. The accelerometers determine acceleration forces along three orthogonal sensitive axes and this data is converted, through integrations, into the aircraft's velocity and position. In a strapdown system in which the accelerometer is fixed in a relation to the geometry of an aircraft, the gyroscopes that measure the aircraft's attitude also measure that of the accelerometer axes. Data measured by the gyros is employed to resolve accelerometer outputs along the appropriate spatially stabilized axes.
Error sources that affect the accuracy of the gyro and accelerometer outputs require compensation to ensure accuracy of the navigation system measurements and functions. Systems and instruments come in various forms and rely upon disparate technologies to produce outputs. Gyroscopes may include gimbaled mechanical or electromechanical arrangements, ring laser and fiber optic arrangements, among others, while accelerometers can be of the pendulous mass type and/or employ piezoelectric or silicon technologies. Regardless, each inertial navigation system arrangement is faced, to a greater or lesser extent, with inaccuracies owing to the error peculiarities of its functional components.
Because inertial grade instruments are required to measure a very large dynamic range of motions, they typically rely on state-of-the-art technologies. These sensors must be able to measure extremely small quantities. For example, a navigation grade accelerometer must measure a few millionths of the standard gravity acceleration, and a gyro must measure a few hundred thousandths of the Earth's rotation rate. Often, it is impossible to precisely identify the sources of minute errors of these magnitudes. Whenever possible, individual error sources should be isolated in order to prevent measurement contamination and to reduce sensitivity to drifts. In particular, gyro error estimation should be rendered insensitive to accelerometer errors since the latter are typically much larger.
One approach of estimating gyro errors independently of accelerometer errors through the use of a calibration model is described in an article by J. Mark et al., in “Fast Orthogonal Calibration of a Ring Laser Strapdown System,” Symposium Gyro Technology, September 1986, Stuttgart, West Germany. Several of the authors of this article are also inventors of the present application. The disclosure of this article is hereby incorporated by reference into the present application. This approach provided for the fast calibration of an orthogonal gyro triad using a state diagram that graphically displayed how 90 degree rotations about each axis of a two axis rate table will generate incremental axis tilts and azimuth errors for the scale factor and orthogonality errors of a gyro triad. Total attitude error changes occur in moving from one position in the state diagram to another position. In order to minimize the effect of accelerometer errors on the gyro calibration process, rotation paths through the state diagram that end back at the starting position were utilized in this approach to eliminate contamination of level tilt observations by constant accelerometer errors.
The fast orthogonal calibration technique and other past approaches provided a method of estimating gyro errors independently from accelerometer errors. However, these past techniques did not account for erroneous outputs which gyroscopes have been found to exhibit under force conditions. The force-dependent gyroscope error could be a contaminant within the calculations in past approaches of estimating scale factor and orthogonality errors of a gyro triad during a calibration procedure.